Known in the art is a discriminating contact sensor which comprises a writing pad comprising a juxtaposed pair of upper and lower sheets each having an electrode attached thereto, separated at a given spacing by a plurality of insulating dot spacers, the upper sheet being flexible, and a position detecting circuit that, when the upper flexible sheet is depressed with an input pen, electrically detects the coordinates (x, y) of the depressed position of the upper flexible sheet (see U.S. Pat. No. 3,911,215).
In this sensor, when the input-side sheet having an electrode attached thereto (upper electrode-bearing sheet) of the writing pad is depressed with an input pen, the input-side sheet is flexed by the writing pressure load to thereby cause the upper and lower electrodes of the writing pad to contact with each other so as to create an electrical connection at the depressed site. This permits detection, by means of the position detecting circuit, of the coordinates (x, y) of the position of the sheet depressed by the tip of the input pen.
A handwritten image input device can be obtained by connecting the above position detecting circuit of the sensor to a storage unit having an appropriate memory capacity. In such a handwritten image input device, the coordinates of the positions of a plurality of dots composing the inputted image are stored in predetermined addresses of the storage unit.
Further, when this storage unit is connected to a display unit, the plurality of position coordinates detected by the position detecting circuit and stored in the storage unit are inputted into the display unit, so that the handwritten images inputted in the writing pad are reproduced on a display screen of the display unit.
In the above conventional writing pad, the upper and lower electrodes are contacted by depressing the upper sheet with an input pen so as to flex the sheet, so that in many cases the distance between the dot spacers is, for example, in the range of 0.635 to 1.9 mm as in the above U.S. Patent or greater.
The inventors' study has revealed that, in the conventional writing pad in which the distance between the dot spacers is large and the inputting is performed by flexing the upper sheet, the difference is slight between the load erroneously applied by contacting fingers or other portions of a writer's hand with the input-side sheet when inputting so as to bring the upper and lower electrodes into contact with each other (erroneous input load) and the load required for inputting characters, etc., with an input pen. Thus, erroneous inputtings occur frequently, causing an erroneous operation.
In order to obviate such erroneous inputtings, the above U.S. Patent has proposed the writing pad wherein a distance between the dot spacers and a height thereof satisfy a predetermined relationship. However, it is difficult to completely avoid erroneous inputtings, even if the proposed relationship is satisfied.
Further, the above writing pad has a drawback in that the flex of the input-side sheet repeated by every input operation may cause disconnection of the electrode formed on the input-side sheet or plastic deformation thereof, so that the upper and lower electrodes may be kept contacting with each other, thereby generating input noise.
Still further, in the conventional writing pad, the electrode formed on the flexible sheet is also moved slightly in accordance with the movement of the input pen upon inputting images to the writing pad, so that friction is generated between the electrodes formed on the pair of sheets at the site where both electrodes are brought into contact with each other upon inputting. Therefore, another drawback of the known writing pad is in that, because such friction is repeatedly generated by every input operation with an input pen, there is a great tendency to damage at least one of both electrodes.
The present invention has been made to overcome the above drawbacks of the prior art, and objects of the present invention are to provide a writing pad which, when characters or graphics are inputted to an input substrate of the writing pad by an input pen, does not cause an erroneous input even when fingers or other portions of a writer's hand erroneously come into contact with the input substrate, and does not cause the erroneous operation even when the input operation is repeated many times, and also to provide a process for producing the same.
Further objects of the present invention are to provide a writing pad in which electrodes formed on substrates are highly resistant to damaging by a vast plurality of repeated sliding inputs with an input pen, and to provide a process for producing the same.